Screw pumps include a pump in which a male thread-like rotor is installed in a fixed stator having a female thread-like inner surface, and the rotor is coupled to a driving shaft via a universal joint (e.g., see FIG. 1 of JP 59-153992 A). This uniaxial eccentric screw pump allows the rotor to eccentrically move with respect to a shaft center of the stator while rotating the rotor by rotating its driving shaft, thereby pumping the fluid from its intake side to the discharge side.
Since in the uniaxial eccentric screw pump utilizing the above-mentioned universal joint, however, the stator is secured and the rotor has to rotate under a large reaction force, friction is likely to occur on an inner surface of the stator. In addition, a pumped fluid is liable to be adhered to the universal joint. What is worse, to wash a dead space of the universal joint, without dissolving the universal joint, it is difficult to clean the dead space.
Therefore, there has been developed a uniaxial eccentric screw pump including a male thread-like rotor directly coupled to a driving shaft without the intervention of the universal joint, and a stator having a male thread-like inner surface, which is rotatably supported by a bearing, and axis of rotation of which is placed eccentrically with respect to that of the rotor (e.g., see FIG. 3 of JP 59-153992 A or FIG. 1 of JP 50-49707 A).